mysql:5.7 I cannot change datadir config

My command:
```docker run -v `pwd`/conf-mysql:/etc/mysql/conf.d -v `pwd`/volume-mysql:/var/lib/db-data -e MYSQL_ROOT_PASSWORD=my-secret-pw  mysql```

My custom config
```
$ cat conf-mysql/my.cnf
[mysqld]
datadir=/var/lib/db-data/
```

Then I expect data in `volume-mysql` but it's empty

In mysql:5.6 works fine

```
$ docker -v
Docker version 17.03.0-ce, build 60ccb22
```